Conventionally, as for binders used for a flat file, letter file or the like, a binder of the type shown in FIG. 15 and FIG. 16 is available. A binder 80 of this type comprises a slender base 81 extending in a plate-like shape, through passages 83 (only one of which is partially invisible in FIG. 6) for binding legs 82, the passages including holes formed at two positions in the longitudinal direction of the base 81, a pair of movable members 85,85 which are provided on the upper side of the base 81 and are capable of sliding respectively between a position where the through passage 83 is opened and a position where the through passage 83 is closed, and an operating section 86 which connects these movable members 85,85 to each other and operates the movable members 85. The operating section 86 is provided with hinge sections 87 of thinner part at the positions located toward the respective edges in the longitudinal direction thereof and at the center thereof, and thereby, capable of bending and transfiguring into a diagonal-brace form as shown in FIG. 16. The binder 80 is adapted so as to, when the operating section 86 is pressed downward to make the respective movable members 85,85 move in the direction that both thereof part away from each other, hold the binding legs 82 in the bent posture and provide binding force thereto; on the other hand, when a piece member 86A protruding from the central area of the operating section 86 is lifted up to make the respective movable members 85,85 move in the direction that both thereof come closer to each other, allow the binding legs 82 to be pulled out from the through passages 83.
However, in the binder 80 as described above, when operating the operating section 86, due to the fact that the frictional resistance between the respective movable members 85, 85 and the base 81 and the binding legs 82 changes, or the fact that the direction of the force given to the operating section 86 changes in the process of operation, the respective movable members 85,85 are prevented from moving evenly resulting in such a difficulty that relative position between the through passages 83,83 and the movable members 85,85 is hardly kept symmetrical. Due to this, the respective binding legs 82,82 are prevented from being given with the binding force or being released therefrom at the same timing resulting in such a problem that unnatural force is applied during operating the operating section 86. Further, since the movable members 85,85 and the operating section 86 are constituted of resin molds in many cases, when the base 81 is removed from the binding legs 83, each of movable members 85 parts away from each other due to the elastic force of the hinge sections 87 blocking up the through passage 83 resulting in such an inconvenience as the relevant movable member 85 prevents the operation of inserting the binding leg 82 though the through passage 83 from being carried out smoothly.